High speed vessels, including military craft, ferries and pleasure boats, are steadily increasing in number in many areas of the world. The development of water jets and light weight construction methods has made it both possible and economical to transport people and goods on the water at increasingly higher speeds. Unfortunately, such higher speeds have also created problems with wake wash in relatively shallow waters near the shoreline such as in harbors, rivers, canals and other estuaries.
Studies have been conducted to determine the effects of running a vessel in shallow waters. One important parameter is known as the depth Froude number, which is a function of vessel speed, the water depth and gravitational acceleration. It has been found that a depth Froude number of unity corresponds to the maximum speed at which free harmonic water waves can travel undisturbed on the surface of a body of water. Vessels operated at a speed in shallow waters which produces a depth Froude number of about unity develop moderate size waves which can travel long distances at high energy. As these high energy waves approach a shoreline, where the water depth continues to decrease, the wave periods become shorter causing the wave height to increase. In turn, these larger waves can be hazardous to other users of the body of water and can severely damage the environment and/or man-made structures along the shoreline.
The speed at which a vessel produces a depth Froude number of unity, for a given shallow water area such as a harbor, river or canal, is known as the critical speed. Modern high speed vessels are operated at subcritical speeds in deep waters, but once entering shallow waters the same vessel speed over ground can be critical or supercritical. The problem of excessive wake wash mainly occurs when a high speed vessel transitions between supercritical speed and subcritical speed in the course of passing through a shallow water area. For example, a high speed ferry must decelerate from supercritical speed to subcritical speed in the course of entering a harbor to unload passengers, and then accelerate from subcritical speed to supercritical speed on the return trip. The longer it takes for the ferry to accomplish these transitions, the more wake wash is created, fuel is wasted and time is lost.
Another problem associated with transitioning between subcritical speed and supercritical speed, particularly for slower vessels, results from the increase in wave making as the vessel approaches critical speed. The larger waves formed by the vessel near the critical speed act, in effect, as a barrier and resist acceleration of the vessel which slows it down. Consequently, additional fuel and energy area required to overcome this wave resistance in the course of accelerating the vessel from subcritical speed through critical speed to supercritical speed.
The problems with vessel operation and unacceptable wake wash noted above have been investigated, but no viable solutions have been proposed. Although a vessel can be operated at reduced, subcritical speed before reaching shallow waters, this substantially increases transport time and can waste fuel. Additionally, while breakwaters have been employed in some areas to reduce the effects of wake wash, this is expensive and often cannot be employed in smaller bodies of water such as river, canals or other estuaries.